Peripheral blood leukocytes (PBL) were drawn from a number of age-matched heterosexual and homosexual men from the Washington, DC area, New York City and Albuquerque, New Mexico. The PBL were sensitized in vitro to influenza virus and to HLA alloantigens. These sensitized cultures were tested for proliferation and for the generation of cytotoxic T lymphocytes specific for influenza virus and alloantigens. Assays were also run for OKT4:OKT8 ratios (i.e., helper: suppressor cell), for thymosin Alpha 1, and for interferon in sera as well as for interferon production in culture in the presence of influenza virus. In the Washington group, approximately 50% of anti-influenza CTL responses were reduced without any detectable loss in reactivity to HLA alloantigens. Abnormalities were also detected in interferon in this group, although all of these donors exhibited normal OKT4:OKT8 and thymosin Alpha 1 levels. A higher proportion of suppressed individuals was observed in the New York group and some of the Albuquerque group was also suppressed. Heterosexuals from all three cities generated CTL responses within the normal range.